Ultrasonic transducer

ABSTRACT

For industrial applications, there is a need for compact ultrasonic transducers which at the same time have a radiation characteristic with small side lobes. This requirement is met by an ultrasonic transducer of conventional design and having a piezoelectric transducer element (1) which is bonded over its main surface (7) to a disk-shaped λ/4 matching element (2), it being the case according to the invention that the circumferential surface (3) of the λ/4 matching element is profiled with a notch (4) of suitable depth (5).

BACKGROUND OF THE INVENTION

The present invention concerns an ultrasonic transducer having adisk-shaped piezoelectric transducer element which is provided with arotationally symmetrical, disk-shaped λ/4 matching element.

An ultrasonic transducer, of the type mentioned above, is described inGerman Patent Publication No. DE 39 11 047 ("the '047 publication"). Inthe transducer of the '047 publication, small changes in the diameter ofthe main surface of a matching element, relative to the diameter of thepiezoceramic transducer element, influence oscillations of thetransducer element to improve the efficiency and radiationcharacteristic of the ultrasonic transducer in conjunction with itssmall dimensions. The '047 publication also discusses that even smallchanges in the shape of the circumferential wall of the matching elementcan substantially change the oscillations of the transducer element. Astraight line, which diverges from, or converges to, the piezoceramicelement is specified as the configuration of the lateral line of thecircumferential surface of the matching element. In this way, thediameter of the main surface of the matching element deviates slightlyfrom the main surface of the piezoceramic transducer element. Dependingon the thickness of the matching element relative to the diameter of thetransducer element, slightly positively or slightly negatively curvedlateral lines are also considered advantageous for the purpose ofachieving a relatively centered high sound pressure.

However, the amplitude distribution occurring in the ultrasonictransducers discussed in the '047 specification has a relative minimumin the central region of the radiation surface. The amplitude rises inthe radial direction, has its maximum at approximately half the radius,and drops off steeply towards the rim (i.e., the edge). This form ofoscillation produces losses in the achievable sound pressure, and theshapes of sound lobes associated therewith have conspicuous side lobes.These losses can lead, in practical use, to faults and malfunctions.

Therefore, the object of the present invention is to provide anultrasonic transducer of the type mentioned above in which, inconjunction with a compact design, a high sound pressure is achievedbecause of an improved form of oscillation with the lowest possiblelosses, and in which the suppression of side lobes is better than -30dB.

SUMMARY OF THE INVENTION

To achieve the above referenced object, the λ/4 matching element has anotch on its circumferential surface and/or on its rear (i.e.,underside) surface facing the transducer element. A particularly goodradiation response is achieved when the notch has a depth of up to atmost, a quarter of the disk diameter of the matching element. Suchultrasonic transducers are particularly suitable for industrialapplications with good acoustic properties and for operations in whichair is the ambient medium.

In an easy-to-produce embodiment, the circumferential surface outsidethe notch has the contour of a regular cylinder. In this case, the notchis subsequently milled, for example, into the circumferential surface ina disk-shaped matching element which is in the shape of a regularcylinder and which is easy to produce.

To achieve a form of oscillation which is effected by few losses, thecircumferential surface has a notch at least of such a depth that, givencircular surfaces of unequal size at the top side and underside of theλ/4 matching element (i.e., given a matching element with two surfacesof unequal diameter and with a circumferential surface shaped as a partof a cone), the notch cuts an imaginary cylinder lateral surfaceprojected into it and proceeding from the smaller circular surface ofthe matching element.

If the piezoelectric transducer element has a main surface of diameter Din a direction of the main radiation of the ultrasonic oscillations, andif the underside circular surface of the λ/4 matching element facing themain surface of the piezoelectric transducer element has a diameter ofbetween 0.9 D and 1.2 D, a particularly effective form of oscillation isrendered possible given the variation in this parameter in conjunctionwith the shape and depth of the notch. The action of the notch withrespect to the acoustic properties is particularly good when the depthof the notch is from 5% to 15% of the disk diameter of the matchingelement.

If the entire ultrasonic transducer, except for the side of the matchingelement facing the medium to be inspected (i.e,. facing away from thepiezoelectric transducer element), is encapsulated in foam,contamination in the region of the notch with the indentations andcorners is avoided. At the same time, in this case the front surface ofthe ultrasonic transducer remains planar, which advantageously permits agood possibility of cleaning in the event of contamination of thetransducer, as well as of its optically improved appearance. If the foamencapsulation comprises polyurethane, the elastic damping of theultrasonic transducer, which damping is a principal target of this foamencapsulation, is exceptionally good. If the ultrasonic transducer isused in air as the ambient medium, the impedance matching problem, whichexists between the piezoceramic transducer element excited intooscillation and air, is advantageously solved when the λ/4 matchingelement comprises syntactic foam.

An embodiment in which a notch on the rear (i.e., underside) surface ofthe matching element is designed as a cylindrical cutout has aparticularly favorable radiation characteristic and is simple toproduce. An equally effective and simple alternative exists when thenotch on the rear (i.e., underside) surface of the matching element isin the form of concentric, annular grooves having a depth of up to atmost half the thickness of the matching element.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in more detail below with the aid ofan exemplary embodiment.

FIG. 1 is a cross-sectional view of an ultrasonic transducer accordingto the present invention.

FIG. 2 illustrates the shape of a sound lobe of the ultrasonictransducer of FIG. 1.

FIG. 3 shows the form of oscillation on the radiation surface of theultrasonic transducer according to FIG. 1.

FIG. 4 is a cross-sectional view of an ultrasonic transducer having arectangular notch on the circumferential surface.

FIG. 5 is a cross-sectional view of an ultrasonic transducer having atrapezoidal notch on the circumferential surface.

FIG. 6 is a cross-sectional view of an ultrasonic transducer having atriangular notch.

FIG. 7 is a cross-sectional view of an ultrasonic transducer having acylindrical cutout on the rear (i.e., underside) surface of the matchingelement.

FIG. 8 is a cross-sectional view of an ultrasonic transducer havingannular grooves on the rear (i.e., underside) surface of the matchingelement.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of an ultrasonic transducer accordingto the present invention having a disk-shaped piezoceramic oscillator 1having a main surface 7 which is bonded to an underside circular surface8 of a rotationally symmetrical, disk-shaped λ/4 matching element 2. Thepiezoceramic oscillator 1 has a diameter D=32.4 mm and a disk thicknessh_(k) =6 mm. The material of the oscillator 1 has a density of 7600kg/m³, an elastic modulus of 65,000 N/mm² and a transverse contractionof 0.29. The λ/4 matching element 2, which has a shape of a regularcylinder, has a rectangular groove 4 on its circumferential surface 3.The rectangular groove 4 has a depth 5 of t_(n) =3.8 mm and a height ofh_(n) =4.5 mm and is therefore shaped as a notch. The groove 4 has aclearance of a_(n) =2.4 mm from the top-side circular surface of thematching element 2, that is, 2.4 mm from the radiation surface of thematching element 2. The disk thickness of the matching element 2 ish_(s) =8.8 mm. The diameter d_(s) of the matching element 2, whichcomprises syntactic foam, matches that of the piezoceramic oscillator 1.The material of matching element 2 has a density of 580 kg/m³, anelasticity modulus of 2150 N/mm² and a transverse contraction of 0.285.

The resultant sound lobe of the ultrasonic transducer according to FIG.1 has the shape illustrated in FIG. 2. As FIG. 2 shows, the sound lobeis virtually free from side lobes. That is, the only side lobes thatoccur have an oscillation amplitude reduced by more than -30 dB withrespect to the main lobe. This exceptionally favorable response is dueto the profiling of the lateral cylinder surface of the matching element2, which entails a mode of oscillation having a virtually idealdistribution of oscillation amplitude on the radiation surface of theλ/4 matching element 2 as shown in FIG. 3. Here, the amplitude isplotted on the ordinate and the longitudinal extent of the radiationsurface, that is, the diameter 4 of the radiation surface, is plotted onthe abscissa.

FIGS. 4, 5 and 6 show further embodiments of the transducer of thepresent invention. In the ultrasonic transducer illustrated in FIG. 4,the notch 4 in the λ/4 matching element 2 is in the shape of a groove aswas the case in FIG. 1. However, the underside circular surface 8 of thematching element 2 projects beyond the main surface 7 of thepiezo-electric oscillator 1. This influences the shape and position ofthe groove 4 which are optimum with respect to the form of oscillation.

In the embodiment of the ultrasonic transducer represented in FIG. 5,the notch 4 in the circumferential surface 3 of the λ/4 matching element2, the matching element 2 being in the form of a regular cylinder, istrapezoidal.

The lateral (i.e., circumferential) surface of the matching element 2,into which the notch 4 is recessed, can also have a conically extendinglateral line. This is shown, for example, by FIG. 6, where the notch 4is triangular in configuration, and the radiation surface of thematching element 2 has a larger diameter than the underside surface 8 ofthe matching element 2, bonded to the main surface 7 of the piezoceramicoscillator 1.

The notches 4 can be configured as a polygon, or else can be designed asround indented shapes. They can be recessed as matching elements 2 incircumferential surfaces 3 of regular cylindrical or conical disks, thediameter of which matching elements is preferably between 90% and 120%of the diameter D at the bonding surface with the piezoceramicoscillator 1 of diameter D.

The exact geometry of the profiling which produces the optimum form ofoscillation according to FIG. 3 depends on the mechanical material dataand the external dimensions of the piezoelectric transducer element 1and of the matching element 2, as a result of which, the order ofmagnitude of the desired operating frequency is also predetermined. Thetransducer must be tuned and optimized anew for each combination ofmaterial data and external dimensions, as well as for the desired formof deflection.

A narrow main sound lobe without side lobes is advantageous in themajority of applications. It is possible, using the lateral notchesaccording to the present invention, to produce, on the radiationsurface, an amplitude distribution of the shape of a Gaussianbell-shaped curve with maximum deviation at the center of the radiationsurface and an amplitude which falls continuously towards the edge.Theoretically, the Gaussian curve is the form of deflection which leadsto sound lobes which are completely free of side lobes. In practice, thetransducers having optimized lateral notches, have extremely weak sidelobes as exemplified in FIG. 2. Depending on the embodiment, it ispossible to achieve a side lobe suppression of -30 to -40 dB.

Gaussian curves of differing edge steepness and a simultaneous change inthe -3 dB width of the main sound lobe can be produced using variousnotch shapes in the lateral surface. In this case, a wider lobecorresponds to a steep drop, whereas a very narrow lobe corresponds to aflatter curve shape. The aperture angles which can be set thereby arebetween approximately 8° and 25°. Due to the Gaussian, equal-phaseoscillation distribution, the transmission coefficient, that is, theratio between the voltage of the received echo signal and the associatedtransmission voltage for a given separation, increases by up to a factorof 5 as compared with an identical transducer without this lateralprofiling.

However, it is also possible, by varying the form of the lateralprofiling, to produce amplitude and phase distributions on the radiationsurface which are other than Gaussian. The sound lobe and thetransmission coefficient can be varied within wide limits to produce a"customized" ultrasonic transducer for specific applications.

The present invention achieves advantageous improvements on thesound-radiating front surface by contouring the lateral surface withnotches 4. The front surface, that is, the sound radiation surfaceitself, remains planar without change in this case, and can easily becleaned when dirty to achieve a good appearance. The ultrasonictransducer can be embedded, except for the radiation surface, in anelastic damping material, preferably poly-urethane. This simultaneouslyprevents contamination of the lateral contour with its indentations andcorners in the region of the notches.

In accordance with the present invention, compact ultrasonic transducershaving a radiation characteristic which is virtually ideal, that is tosay free from side lobes, can be simply produced. This is achieved withconventional components for ultrasonic transducers by profiling thecircumferential surface of the matching element with a notch of suitableshape and depth.

In accordance with the FIGS. 7 and 8, the radiation response of theultrasonic transducer can be improved, not only by contours on thecircumferential surface 3 of the matching element 2, but also by notches9, 10, 11 on the rear (i.e., underside) surface 8 of the matchingelement 2 facing the piezoceramic oscillator 1.

A cylindrical cutout 9 is provided on the rear (i.e., underside) surface8 in FIG. 7. In the ultrasonic transducer represented in FIG. 8, thenotches on the rear (i.e., underside) surface 8 of the matching element2 comprise concentric, annular grooves 10, 11. A particularly favorableradiation response can be achieved by combining lateral notches andnotches at rear profiles of the matching element.

We claim:
 1. An ultrasonic transducer comprising:a) a piezoelectricdisk-shaped transducer element having an end face; and b) a rotationallysymmetrical, disk-shaped λ/4 matching element, the matching elementi)having an underside surface facing, and provided on, the end face of thepiezoelectric transducer element, ii) having a diameter, iii) having acircumferential surface, and iv) having at least one of acircumferential notch provided on the circumferential surface and aconcentric notch provided on the underside surface, wherein any notchprovided on the circumferential surface of the matching element has adepth of up to 1/4 of the diameter of the matching element and extendscompletely around the circumference of the matching element, and whereina fractional surface of the underside surface covered by any notch inthe underside surface of the matching element is smaller than the endface of the transducer element.
 2. The ultrasonic transducer of claim 1wherein the circumferential surface of the matching element correspondsto a lateral surface of a cylinder.
 3. The ultrasonic transducer ofclaim 1, wherein the matching element has a radiation surface having adiameter differing from the underside surface of the matching elementwhereby the circumferential surface of the matching element is shaped asa part of a cone, and wherein the circumferential surface has a notchwhich is at least of such a depth that the notch breaks an imaginarylateral cylinder surface projected from the smaller of the radiationsurface of the matching element and the underside surface of thematching element.
 4. The ultrasonic transducer of claim 1 wherein thepiezo-electric transducer element has a diameter D, andwherein theunderside surface of the matching element facing the transducer elementhas a diameter of between 0.8 times D and 1.2 times D.
 5. The ultrasonictransducer of claim 1 wherein the depth of the notch is 0.05 to 0.15times the diameter of the matching element.
 6. The ultrasonic transducerof claim 1 wherein the matching element includes a radiation surface,the ultrasonic transducer further comprising foam, said foamencapsulating the entire ultrasonic transducer, except for the radiationsurface of the matching element.
 7. The ultrasonic transducer of claim 6wherein the foam encapsulation essentially comprises polyurethane. 8.The ultrasonic transducer of claim 1 wherein the λ/4 matching elementcomprises syntactic foam.
 9. The ultrasonic transducer of claim 1wherein a notch on the underside surface of the matching element is acylindrical cutout having a depth of up to half the thickness of thematching element.
 10. The ultrasonic transducer of claim 1 wherein anotch on the underside surface of the matching element exists in theform of concentric, annular grooves having a depth of up to half thethickness of the matching element.